1. Field of the Invention
The present invention relates to a picture quality monitoring apparatus, in particular to a picture quality monitoring apparatus that is suitable for monitoring a picture quality by comparing two pictures or for remote-monitoring a picture quality degradation collectively at a center by comparing pictures at various points on a transmission link that includes a plurality of transmission processing units in tandem.
2. Description of the Related Art
Conventionally, where a picture quality is monitored by comparing two pictures, there is a method for determining a picture quality degradation based on PSNR obtained between the corresponding pictures. For example, where a picture quality is monitored by comparing a processed picture with an original picture, the original picture and the processed picture are taken into a monitoring apparatus, and PSNR of the processed picture with respect to the original picture is obtained frame by frame. In the meantime, it is monitored whether or not PSNR decreases.
Where the video is based on NTSC, the video is composed of 30 pictures (frames) per second. Therefore, if PSNR of each frame is in an attempt to be obtained, calculation of PSNR will be carried out thirty times per second, and PSNR can be calculated by the following Equation (1).
[Equation 1]PSNR=10 log10(2552/MSE)   (1)
Herein, MSE can be expressed by the following Equation (2) where it is assumed that b(x, y) is a signal (8 bits) of an original picture, bp(x, y) is a signal of the corresponding processed picture, and N is a total number of pixels of a picture.
                    [                  Equation          ⁢                                          ⁢          2                ]                                                            MSE        =                              (                          1              /              N                        )                    ⁢                                    ∑                              (                                  x                  ,                  y                                )                                      ⁢                                                  ⁢                                          {                                                      b                    ⁡                                          (                                              x                        ,                        y                                            )                                                        -                                                            b                      p                                        ⁡                                          (                                              x                        ,                        y                                            )                                                                      }                            2                                                          (        2        )            
The inventors have proposed a transmitted picture quality monitoring apparatus in Patent Documents 1 and 2, which remotely monitors a picture quality degradation collectively at a center by comparing pictures at various points on a transmission link that includes a plurality of transmission processing units in tandem.
In the transmitted picture quality monitoring apparatus according to Patent Document 1, pictures extracted and picked up from two points on a transmission link are, respectively, divided into blocks, each block having a certain size, the picture of each block is orthogonally transformed, the transformed coefficient is extracted, and is transmitted to a central monitoring room as a picture quality characteristic amount. In the central monitoring room, the picture quality characteristic amounts transmitted from the two points are compared with each other to estimate PSNR. And, if PSNR is reduced, it is judged that the picture quality degradation occurs between the two points, and an alarm is issued.
In the transmitted picture quality monitoring apparatus according to Patent Document 2, the picture quality characteristic amounts are extracted by using a orthogonal transformation and PN-series multiplication, and the picture quality characteristic amounts are transmitted to a central monitoring room.    [Patent Document 1] Japanese Published Unexamined Patent Application No. 2003-87823    [Patent Document 2] Japanese Published Unexamined Patent Application No. 2003-9186
The transmitted picture quality monitoring apparatuses according to Patent Documents 1 and 2 are on the assumption that two pictures which are subject to comparison are transmitted, as they are, without varying the band in midstream. That is, if the band of two pictures, which are the objects of comparison, is the same, it is possible to determine differences between both pictures by extracting the picture quality characteristic amounts of the corresponding pictures and by comparing them.
However, there may be a case where the band of the transmitted pictures is narrowed by transmission and processing on the transmission link. In particular, where D/A conversion is included in the transmission link or D/A conversion is performed on the way of the transmission link, there are many cases where the band of the D/A-converted analog signal is further narrowed than the band of the digital signal of the original picture before D/A conversion.
For example, in a case where the original picture is based on a digital signal (a digital component signal) of D1 format, and the original picture is converted to a NTSC analog signal (an analog composite signal), with regard to the luminance signal, the band of the before-conversion digital signal is 6 MHz, and the band of the converted analog signal is 4 MHz. In detailed processing for the case where the digital signal of D1 format is converted to the NTSC analog signal, a low-pass filter is applied to the digital luminance signal having a band of 6 MHz, and the band is limited to 4 MHz, and chrominance signals having a band of 3 MHz is also limited with respect to the band. And, the band-limited luminance signal and chrominance signals are multiplexed.
In this case, the digital signal whose horizontal bandwidth is 6 MHz will be converted to the analog signal whose horizontal bandwidth is 4 MHz. Therefore, the converted analog signal is the signal whose bandwidth is considerably limited with respect to the before-conversion digital signal.
Herein, if PSNR is obtained by merely comparing the picture quality characteristic amounts of these two pictures with each other, PSNR is lowered due to influence of high-frequency component attenuation caused by the band limitation of the low-pass filter, and PSNR does not express a true picture quality degradation in a transmission link for which monitoring is required. Therefore, there remains a problem by which it is not possible to monitor the true picture quality degradation only by using the transmitted picture quality monitoring apparatuses according to Patent Documents 1 and 2 as they are.